The deleterious health effects that result from long-term tobacco addiction extract an enormous socioeconomic cost each year, yet astonishingly little is known about the causes of tobacco, or more appropriately, nicotine addiction. Nicotine use and/or treatment elicits a broad array of behavioral and physiological effects that presumably arise subsequent to the binding of nicotine to brain nicotinic cholinergic receptors (nAChR). Brain nAChRs are ligand-gated ion channels that are readily activated and desensitized by nicotinic agonists. Molecular approaches have identified 10 genes that encode for nAChR subunits. The mRNAs for some of these subunits are expressed in many brain regions whereas others appear to be expressed in only a few. The studies outlined in this proposal will attempt, using neurochemical and genetic strategies, to identify the subunit composition of nAChRs that make up several different ligand binding sites and regulate presynaptic processes such as ion flux and neurotransmitter release. These studies will make use of several null mutant (knockout) mice that have specific nAChR subunit genes disrupted. Other studies will assess the effects of chronic nicotine treatment on the number and function of mouse brain nAChRs. Experiments done with cell lines expressing neuronal-type nAChRs have demonstrated that chronic nicotine treatment elicits a permanent inactivation of receptor function. The chronic studies outlined in this proposal will evaluate whether a similar phenomenon occurs in mammalian brain, and whether this effect varies across brain regions and receptor subtype. The information provided will also be useful in testing the hypothesis that changes in nAChR number and function underlie tolerance and dependence on nicotine.